1. Field of Invention
The present invention relates to an apparatus useful to controllably volatilize and mix a volatile anesthetic, having vapor pressure of about 760 mm of mercury at about 20.degree. C. into an effective anesthetic/diluent gas mixture to be administered to a mammal. More specifically, the invention is an apparatus to volatilize an inhalation anesthetic of structure, CF.sub.2 H--O--CFH--CF.sub.3, I--653, and a diluent gas, to a human being.
2. Description of Related Art
Anesthetic vaporizers of the art useful to vaporize currently used and experimental liquid inhalation anesthetics having low vapor pressures:
______________________________________ Anesthetic B.P., .degree.C. Vapor Pressure (at .degree.C.) ______________________________________ Halothane 50. 243 Torr Methoxyflurane 104.65 23 Enflurane 56.5 175 Isoflurane 48.5 238 (but not including) I-653 (Experimental) 23.5 664 ______________________________________
The vaporizers of the art act for liquid inhalation anesthetics as "humidifiers" to vaporize and entrain the vaporized anesthetic. These machines come in a number of configurations, e.g., variable bypass vaporizers, measured flow vaporizers, bubble-through vaporizers, etc.
Gaseous inhalation anesthetics, such as nitrous oxide, cyclopropane, ethylene etc., are delivered satisfactorily and accurately using only a flowmeter, and generally do not need to be heated.
I--653 because of its boiling point liquifies or vaporizes so easily at ambient temperature, that the accurate delivery of the anesthetic as a gas is very difficult or impossible using the above conventional vaporizers.
An overview of the vaporizers used in inhalation anesthetics is found in "Vaporizers, Chapter 4", of Understanding Anesthesia Equipment: Construction, Care and Complications, Second Edition, by J. A. Dorsch and S. E. Dorsch, published by Williams and Wilkins of Los Angeles, Calif. in 1984, pp. 77-135.
Dorsch et al. describe various methods used to supply heat to keep the temperature of the volatile anesthetic liquid constant for accurate delivery. These methods include the following:
(a) Construct the vaporizer of a large mass of copper to conduct heat easily from ambient air or operating tabletop. PA1 (b) Using wicks in direct contact with metal (increases the surface contact only). This does not aid in temperature control but only increases the efficiency of vaporization. PA1 (c) Use of a water jacket--but this method proved to be rather inefficient. PA1 (d) Using an electric heater inside the vaporizer. This method adds problems which make its practical use complicated so that no anesthetic vaporizers with this feature are currently being produced. PA1 (a) a housing effective to thermally insulate the interior chamber of the apparatus from the operating room environment; PA1 (b) a diluent gas supply; PA1 (c) an anesthetic gas supply located within the space created by the housing; PA1 (d) a mixing chamber within the space created by the housing which has PA1 (e) at least one heating means to maintain the constant temperature of the components within the space created by the housing of between about 30.degree. and 50.degree. C. within plus or minus 1.degree. C., and PA1 (f) at least one sensing means within the space created by the housing to accurately monitor and maintain the internal temperature of the space enclosed by the housing. PA1 (a) maintaining a volatile anesthetic having a vapor pressure of about 760 mm of mercury at a temperature of between about 15.degree. and 30.degree. C.; PA1 (b) accurately measuring an effective amount of the vapor of the anesthetic of step (a) in a flowmeter; PA1 (c) accurately measuring an effective amount of diluent gas independently selected from the group consisting of air, oxygen, nitrogen, nitrous oxide or mixtures thereof; and PA1 (d) combining and mixing the volatile anesthetic of step (b) and the diluent gas of step (c) in a mixing chamber to produce an effective anesthetic vapor mixture to administer to a mammal, wherein in steps (a), (b), (c) and (d), all vapor components are maintained at the same temperature of between about 30.degree. and 50.degree. C., accurate to plus or minus 1.degree. C. PA1 (a) a housing effective to thermally insulate the interior components of the apparatus from the operating room environment; PA1 (b) an diluent gas supply; PA1 (c) an anesthetic gas supply located with the space created by the housing; PA1 (d) a mixing chamber within the space created by the housing which has: PA1 (e) accurate heating means to maintain a constant temperature within the space created by the housing of between about 35.degree. and 50.degree. C. within plus or minus 1.degree. C.; and PA1 (f) thermal sensing means within the housing to accurately monitor and maintain the temperature of the components within the space enclosed by the housing.
A similar discussion of the art is summarized by A. B. Dobkin and A. R. Hunter in Chapter 11; "Vaporizers for Inhalation Anesthetics," in Development of New Volatile Inhalation Anesthetics, Dobkin (ed.) published by Elsevier/North-Holland Biomedical Press, in 1979, pp. 311-341.
Some U.S. patents of general interest, for example: W. Jones, U.S. Pat. No. 3,575,168; H. G. Breiling, U.S. Pat. No. 3,651,805; W. H. Hay, U.S. Pat. No. 3,703,1772; U. Sielaff, U.S. Pat. No. 3,841,560; P. Seidel, 4,017,566; P. Seidel, U.S. Pat. No. 4,075,297; S. Albarda, U.S. Pat. No. 4,587,066; and T. R. Clapham, U.S. Pat. No. 4,807,634.
United Kingdom Pat. No. 1,307,905 issued to S.A.C.C.A.B., S.P.A., an Italian company, is also of general interest.
More specifically, H. G. Epstein, et al disclose the "Oxford Vaporizer No. 2, in The Lancet, pp. 64-66, in the issue dated Jul. 19, 1941. This inhalation anesthetic vaporizer uses the latent heat of fusion of an organic compound, e.g. para-dichlorobenzene (m.p. +52.degree. C.), to maintain a constant temperature for the anesthetic.
S. R. Wilson et al. disclose a "New Invention-A Warm Ether Bomb" in The Lancet, published Feb. 12, 1921, p. 366. This apparatus simply heats the liquid ether so that it will self delivery as a warm gas to the patient. The apparatus needs to withstand a pressure of about 250 pounds per square inch, and the anesthetic gas is delivered to a face mask at between 90.degree. and 98.degree. F.
All of the references cited in this application are incorporated herein by reference.
After examination of the above patents and articles, it is apparent that none of the references, individually or collectively, teach or disclose a method of precisely controlling the vapor of a volatile anesthetic having a vapor pressure about one atmosphere at about 15.degree. to 30.degree. C. (The temperature range present in most operating rooms.)
In the evaluation of a new inhalation anesthetic (having a vapor pressure of about one atmosphere) (e.g. CF.sub.2 H--O--CFHCF.sub.3), called I--653 by its producer, Anaquest (a subsidiary of British Oxygen Corporation) of 2005 W. Beltline Highway, Madison, Wis. 53713), it became rapidly apparent that the conventional inhalation anesthetic vaporizers could not be directly used. To function in any reasonable manner, required extensive modifications to produce the medically required controllable concentration of gaseous anesthetic. In a warm room (in excess of 24.degree. C.), I--653 boils thus producing a 100% concentration of gaseous anesthetic. In a cool room (e.g. 20.degree. C. or less), vaporization of I--653 using conventional inhalation anesthetic vaporizers produce a vapor concentration that varies enormously depending upon the temperature. Even a temperature change of 1.degree. C. can produce a significant vapor change which is unacceptable in a hospital operating room environment. Further, excessive amounts of the costly I--653 anesthetic can be used and lost. The present invention provides an apparatus and such a method of administration that overcomes these problems.